1. Field of the Invention
The present invention relates to a method for transmitting downlink control information and a method for generating a codeword for the same.
2. Discussion of the Related Art
Among fundamentals of basic code theories, a few things necessary for a description of the present invention will now be explained.
When a typical binary error correction code is represented as [n, k, d], ‘n’ means the number of bits of an encoded codeword, ‘k’ means the number of information bits before encoding, and ‘d’ means a minimum value among distances between codewords. Here, since the codeword is a binary code, the length of the codeword is 2n and the total number of the encoded codewords is 2k. On the other hand, the binary error correction code may be simply expressed as an [n, k] code for convenience.
Hereinafter, ‘n’, ‘k’ and ‘d’ will have the abovementioned meanings unless otherwise stated.
A code rate R is defined as a value dividing the number of information bits by the number of bits of a codeword (i.e., R=k/n).
A Hamming distance is the number of bits of which corresponding bit values are different in binary codes having the same number of bits. If a Hamming distance ‘d’ is equal to or greater than (2a+1), errors as many as ‘a’ can be corrected. For example, two codewords are ‘101011’ and ‘110010’, a Hamming distance between the two codewords is 3 and one error can be corrected.
A minimum value among distances between any two codewords belonging to a code is defined as a minimum distance. The minimum distance is one of important metrics to evaluate performance of a code. The aforementioned Hamming distance can be used as a distance between two codewords. As a distance between codewords generated through an encoding process becomes greater, since the probability that a corresponding codeword is judged to be a different codeword becomes lower, encoding performance is increased. The performance of a code is determined by a distance between codewords having the worst performance, i.e., a minimum distance between codewords. Consequently, a code in which minimum distance is maximized shows good performance.
Meanwhile, a 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) system has proposed that information indicating a format of a control channel during transmission of control information be transmitted through a physical control format indicator channel (“PCFICH”). The PCFICH demands to transmit a code having a very low code rate to minimize an occurrence of an error during transmission since information on a format transmitting control information is transmitted therethrough.
However, in generating a long-length code having a very low code rate, it is difficult to set a minimum distance between codewords to a maximum value.